Solvent extraction processes are commercially practiced in the areas of lube oil and transformer oil upgrading to remove undesirable aromatics. In the practice of such extraction processes, the selective solvent used is recovered from the extract and raffinate phases by distillation and stripping which are energy intensive.
U.S. Pat. No. 3,566,991 sought to improve the recovery of aromatic products by employing a dialysis membrane separation technique. In U.S. Pat. No. 3,566,991 aromatics are separated from a mixture of aromatics and non-aromatics. The specification teaches a process which involves the steps of (1) using a selective solvent to obtain a solvent rich aromatics extract phase and a solvent lean non-aromatics raffinate phase, (2) passing the extract phase to a first cell of a dialysis chamber containing at least 2 cells separated by a membrane permeable to the aromatics component of the extract phase thus the solvent permeates freely without an applied pressure, (3) removing solvent from the first cell and recovering aromatics from the second cell and (4) recycling at least a portion of the recovered solvent after stripping to the extraction zone. No flux, flow or purity data is presented. The key to this patent is the unique aromatics separation via a dialysis membrane process. Basically the process is used to increase the yield of the extraction process using dialysis in addition to distillation. The respective solvent from each product was separated/recovered by distillation and not by means of a membrane.
U.S. Pat. No. 3,725,257 deals with a process of separating aromatic hydrocarbons from hydrocarbon mixtures. The mixture of liquid hydrocarbons is extracted in the liquid phase with ethylenediamine solvent containing 0-20 wt% water. The extract phase comprises predominantly aromatic hydrocarbons, solvent and a minor amount of non-aromatic hydrocarbons. The raffinate phase comprises predominantly non-aromatic hydrocarbons. Each phase is separately collected. The extract phase is cooled to a temperature lower than the temperature of the extraction step and then separated into a heavier solvent containing phase and a lighter hydrocarbons containing phase which is further processed. It must be noted that the separation of the extracted phase into separate solvent and hydrocarbon phases is effected by cooling and decanting and not membrane separation. The heavier solvent separates on cooling and is recycled to the extraction zone. In a specific embodiment this solvent recycle is to a point intermediate between the feeding point of the ethylenediamine selective solvent and the feed point of the mixture of hydrocarbons to be extracted.